Recent advances in circuit manufacturing technology have spawned increases in circuit complexity while simultaneously allowing for production of smaller and smaller circuits. For example, with the advances in "bump" or "flip chip" technology, electrical interconnections, which had previously been on the order of 0.020 inches between centers, are approaching 0.001 inch (25 microns) between centers. In addition, as the manufacture of circuit boards becomes ever more complex due to increasing lead count, decreasing lead pitch, and the switch to double-sided construction, it is important that the manufacturing engineer obtain real-time data about the circuit manufacturing process.
Presently, the best circuit board production lines produce assemblies with a solder joint defect rate of about 10-50 parts-per-million (PPM). Thus, as the number of solder connections on a circuit board increases, it becomes increasingly important to reduce joint solder-defect levels through process control feedback, and circuit board inspection. For example, for a circuit board design having 100,000 solder connections, and a joint defect level of 10 PPM, less than 40% of the circuit boards produced will be defect free.
The trend in manufacturing is toward more joints per board. There are boards being designed with over 150,000 connections on a single small board. In addition, advanced techniques are being used to increase the complexity of interconnections on integrated circuit (IC) chips as well. Therefore, additional means must be employed to further reduce the process defect level. Even with further process improvements, statistical process control, and closed loop feed-back, many products will still require an inspection system to reduce the remaining defect levels to acceptable rates for product shipment.
It has become apparent that, with increased circuit complexity and decreased circuit size, visual inspection by subjective human inspectors has become inadequate. Consequently, the circuit manufacturing industry has sought to develop an automated circuit inspection system which is capable of meeting the needs of present circuit board manufacturers. Inspection systems in the past have met with limited success for through hole technology boards and single-sided surface mount technology (SMT) boards. The most successful of these has been automated transmission X-ray. For example, U.S. Pat. No. 4,809,308 by Adams, et al., discloses an automated transmission X-ray device for performing circuit board solder quality inspections. It has been found, however, that automated transmission X-ray exhibits additional problems with double-sided SMT boards due to the overlapping interference of the images produced by the top side components with the images produced by components on the bottom side of the circuit board. That is, when an X-ray beam penetrates through two separate connections on both sides of a circuit board, the image formed on a detector is a composite image of both connections. This could present serious problems when attempting to analyze each connection individually. Because of the shortcomings exhibited by past automated inspection systems, a new automated circuit inspection technology was needed.
Recent developments in scanned-beam laminography (SBL) have provided improved resolution and accuracy in the inspection of electronic devices, particularly for high component density and double-sided circuit boards. By laminographically scanning an electrical connection, a cross-sectional image of the electrical connection can be produced which significantly reduces the overlapping image interference exhibited in transmission X-ray inspection systems. The introduction of SBL for the automated inspection of circuit boards has provided a means for analyzing individual connections on high density and double-sided circuit boards. Automated SBL typically provides for increased image resolution without requiring the complex mechanical operations that are typical of many automated inspection systems. Thus, automated SBL has proven to be superior for inspection of high density circuitry, and for applications which require the inspection of multiple layers within an object. Such an automated laminographic inspection system which produces cross-sectional images of electrical connections on a circuit board is described in U.S. Pat. No. 4,926,452, issued May 15, 1990.
Until now, SBL automated circuit inspection systems have been suitable for uses such as the inspection of solder connections on circuit boards. However, with the advent of integrated circuit designs wherein the entire circuitry that would normally be placed on a circuit board is instead deposited onto a silicon substrate, higher resolution inspection systems than had been previously contemplated have become a priority. With chip-level connections in the sub-micron range (i.e., 1.0 microns and smaller), the current technique of circuit inspection is not very practical. Thus, a need exists for a high-resolution, automated circuit inspection system which is capable of inspecting interconnections and circuitry (e.g., within an integrated circuit) using a resolution sufficient to analyze connections in the sub-micron range.